The present invention relates to a method and apparatus for preparing picture masks, and particularly for preparing a fine mask of a boundary on a picture of an area of interest to be separated from the remainder of the picture.
The purpose of mask preparation is to "cut" an area of interest and to extract it from its background. When cutting a mask, the contour line should be as smooth as possible. In addition, only the area of interest should be in the mask, to ensure that when that area is cut and placed on a different background, there is no "noise" resulting from the original background. It is desirable that the resolution of a mask be at least as high, or higher, than the resolution of the original picture.
At the present time, the mask of a multi-colour picture is generally prepared manually in the following manner: the separation photograph of the multi-colour picture, on which the area of interest is the most obvious, is placed on a light table, and that area is painted with an opaque paint. The resulting film is then photographed to obtain the mask area, which is photographed again together with each separation to obtain the final picture. Masks are also prepared by the use of computerized systems, which provide various tools for this purpose. One tool used in mask preparation is the polygon, or smooth mask. In drawing this type of mask, the operator draws a precise contour of the masked area. However, this procedure is relatively slow since the operator has to be accurate work. Although speed increases with operator experience, the contour is only as good as the operator's accuracy.
Another tool provides the operator with the means to add a pixel to a range of CMYK (cyan, magenta, yellow, black) values that determine the masked pixels. This is a fast and accurate technique, but cannot be used in most cases since the CMYK values of the area of interest within and outside the mask overlap. In addition, since the resulting map has only raster information and not vector information, its resolutions cannot be higher than the original picture. Most of the time, masks prepared using this tool need to be retouched in order to smooth boundaries and clean the mask.
Both of the above tools are used when the picture is displayed in maximum resolution.
It can thus be seen that mask preparation according to the above existing procedures is very time-consuming particularly if good quality masks are to be produced.
A number of techniques have been described in the patent literature to improve the above existing procedures.
According to the procedure described in DE2920070 (Israel Patent 60083), an examination zone of predetermined width is specified and the coordinates of the image dots located within the examination zone are determined; the image dots which belong to those coordinates for their tone value, and the precise coarse of the contour is determined from the tone value differences between the individual image dots.
Another technique as described in U.S. Pat. No. 4,884,224, approximate information regarding the position of the contour on the original film is processed with image sensor data in order to calculate an absolute contour line position. According to a still further technique described in Duenyas and Yad-Shalom Israel Patent Application 88286 (assigned to the same assignee as the present application), the edge segments of the picture are produced by examining rectangular sections of variable width, length and orientation.